Problem: A circle has a circumference of ${20}$. It has an arc of length $4$. What is the central angle of the arc, in degrees?
Explanation: The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$. $\dfrac{{\theta}}{360^\circ} = \dfrac{{s}}{{c}}$ $\dfrac{{\theta}}{360^\circ} = {4} \div {20}$ $\dfrac{{\theta}}{360^\circ} = \dfrac{1}{5}$ ${\theta} = \dfrac{1}{5} \times 360^\circ$ ${\theta} = 72^\circ$ ${20}$ ${4}$ ${72^\circ}$